A two-phase model for growth of most solid tumors has been proposed; the avascular phase and the vascular phase. During the avascular phase, growth is limited to a small population of tumor cells (approximately 10 to the 6th power). Tumors escape this avascular phase by secreting an angiogenesis factor (TAF) that induces the growth of new blood vessels. Current work in our laboratory is directed toward developing in vitro assays for TAF, and testing a cartilage-derived angiogenesis inhibitor in tumor bearing animals.